chore(bedrock-agentcore): support A2A protocol for runtime (#35888)

Newly supported protocol.

https://docs.aws.amazon.com/AWSCloudFormation/latest/TemplateReference/aws-resource-bedrockagentcore-runtime.html#cfn-bedrockagentcore-runtime-protocolconfiguration

### Checklist
- [x] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

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*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: mazyu36

packages/@aws-cdk/aws-bedrock-agentcore-alpha/agentcore/runtime/types.ts

@@ -16,4 +16,9 @@ export enum ProtocolType {
    * HTTP protocol
    */
   HTTP = 'HTTP',
+
+  /**
+   * A2A protocol
+   */
+  A2A = 'A2A',
 }